The concept of 'moments of inertia' doesn't just apply to things on a car that spin. Think about the entire car itself. We've all heard that, generally speaking, mid-engine vehicles corner better. We've all noticed the shorter bumper overhangs-and have you opened a new car's hood lately? It seems like the vehicle's front quarter is just empty space.
Trying to rotate a car is the same as spinning a wheel or brake disc around an axle. The further the mass is situated from the axis of rotation, the more difficult it is to rotate. Think of a barbell compared to a dumbbell of equal weight. Hold a 25-pound dumbbell and rotate your wrist back and forth... fairly easy. Do the same thing with a 25-pound barbell and it's a different story. It's tougher to get it rotating and also much more difficult to send it in the opposite direction. Same amount of weight, just a higher moment of inertia.
It's the same with a car. If all the weight is spread out at the ends, it becomes harder to rotate in a turn. It also explains why a poorly driven rear-engine car will fling itself off the outside of a turn like a flying hammer (Like a Flying Hammer... that will be my first album). The downside to a mid-engine car is that it will rotate fast, so once it starts to spin, it can all go bad very quickly. There's a reason why NSXs and Boxsters aren't tearing up the drift circuit.
Hopefully this will give a better understanding of why your trusted tuner is trying to talk you out of that plus-four wheel fitment you want so badly. If not, when you're in a bar with your buddies and my band's new hit single is played, you can be the cool guy with the story behind our name. If nothing else, maybe I can at least get you a free beer.